The present invention relates to time domain induction systems and methods, and more particularly to time domain induction systems and methods for identifying buried objects, which includes objects that are at least partially buried.
A precise map of the subsurface avoids damaging existing utilities, such as water, gas, and electric lines, during excavation. For example, prior to digging trenches to install new pipes, a construction crew must know where the existing pipes are buried to avoid damaging them. A lack of accurate maps of construction sites results in thousands of broken pipes and billions of dollars in repair costs.
Conventional pipe location equipment requires an operator to connect a transmitter directly to a pipe. After making the connection, the conventional equipment injects a single frequency current into the pipe at a location where the pipe is exposed (for example, by connecting to a water pipe at a fire hydrant). The conventional equipment then measures a resulting magnetic field on the ground surface. Such conventional equipment cannot locate pipes that are not at least partially exposed above ground because direct connection to the pipes are not possible. Furthermore, these conventional devices can determine the depth and direction of a pipe when only single pipes are running through a site. If multiple pipes are buried in a field of interest, these conventional devices cannot properly locate them.
Other types of conventional utility locating devices, which do not require direct connection to an exposed portion of a pipe, include wave propagation systems, such as ground penetrating radar (GPR) equipment. GPR-based wave propagation utility location systems tend to work well when soil conductivity is relatively low, but do not work as well when conductivity is high because the waves attenuate rapidly through the soil prior to reaching the pipes.
Another type of system is suitable for use in mining applications in conductive soil to locate buried objects. These devices, however, are designed to detect conductive ores, aggregates, aquifers, bedrock, and buried waste, but such systems are not suitable for locating buried pipes. One reason why such conventional systems are not suited for locating buried objects such as pipes is that the geometry of the transmitting antennas in these systems does not allow the systems to couple effectively with buried objects having the geometry of pipes. For example, a conventional system described in U.S. Pat. No. 5,654,637 to McNeill entitled “Method for Detecting Buried High Conductivity Objects Including Scaling of Voltages for Eliminating Noise of a Particular Depth” (“the '637 patent”), has one square transmitting loop and two square receiving loops placed at different heights. The system described in the '637 patent estimates the depth of buried objects, such as barrels, from a ratio of the output voltages of the receiving loops. The system computes this depth estimate by modeling the buried object as a sphere. The sphere model used in conventional systems is not appropriate for many kinds of buried targets, including pipes, because the field emitted by a sphere is different from the field emitted by an elongated object such as a pipe. Moreover, a single square transmitting loop does not couple to pipes that lie directly beneath it because the total induced current through a cross section of the pipe is zero and the emitted magnetic field decays rapidly away from the pipe. Accordingly, conventional systems are severely limited in their ability to detect buried objects such as pipes.
A time domain induction system consistent with the present invention can overcome the disadvantages of conventional systems. Further, a time domain induction system consistent with the present invention can fit onto a moving vehicle, trailer, or portable housing so that subsurface images can be formed as the system is moving.